Laser diodes are used in many applications such as laser printing, optical data storage, and optical fiber communications. Laser diode structures usually include a central waveguide/quantum well “active” region flanked by upper and lower cladding layers. The active region or “core” includes multiple quantum well structures that together form a high optical gain region where lasing occurs. An upper cladding layer above the core and a lower cladding layer below the core have refractive indexes lower than the core refractive index. The cladding lower refractive indexes form a waveguide effect that contains the optical modes in the core. The guided optical modes propagate along the active region creating a laser light beam that emanates from the laser structure.
One method of creating the index of refraction differential that confines a transverse optical mode in the active region utilizes a GaN waveguide layer and an AlGaN cladding layers. Conventional III-V compound nitride lasers often use either a short period AlGaN/GaN superlattice layer or a bulk AlGaN layer doped with Mg as an upper cladding layer. However, the large lattice mismatch between GaN and AlN limits the maximum thickness and alloy composition of an AlGaN cladding layer. The alloy composition limits prevent large refractive index differentials between cladding layer and active region. Reduced refractive index differentials adversely affects optical confinement.
Besides guiding optical modes, a second objective of cladding layer design is to facilitate electrical current flow through the laser structure. In order to facilitate current flow through the laser, the cladding layer is typically doped. Magnesium is often used to p-dop the AlGaN alloy cladding layer. However, high hole concentrations are difficult to achieve because increasing the AlN mole fraction of the AlGaN alloy increases the ionization energy of the Mg acceptors. Low hole concentration result in high device resistance and increased Joule heating during device operation.
Eliminating Aluminum in cladding layers avoids the Mg and Al conflict. However, eliminating aluminum results in GaN:Mg cladding structures that provide weak optical mode confinement and allows transverse optical mode penetration into a p-metal layer that serves as an electrical contact. Absorption loses for a p-metal is expected to be greater than 100 per centimeter, even for a laser structure with a 700 nm thick GaN:Mg cladding layer. Such high modal losses significantly exceed the distributed loss in a conventional laser and lead to an increase in threshold current for the laser. Very high modal losses can quench laser operation.
Thus an improved method or material for confining the optical modes is needed.